TB14: Liquid Crystal Display
(LCD)
Overview
 LCDs are used in digital clocks, cellular phones, desktop and laptop
computers, and some televisions and other electronic systems.
 They offer a decided advantage over former display technologies, such
as cathode ray tubes,
in that they are much lighter and thinner and consume a lot less power
to operate.
 LCD technology relies on special electrical and optical properties of a
class of materials known as liquid crystals,
first discovered in the 1880s by botanist Friedrich Reinitzer.
JHLin, Applied EM; LCD
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Physical Principle
 Liquid crystals are neither a pure solid nor a pure liquid, but rather a
hybrid of both.
One particular variety of interest is the twisted nematic (向列相)
liquid crystal whose rod-shaped molecules have a natural tendency to
assume a twisted spiral structure
when the material is sandwiched between finely grooved glass
substrates with orthogonal orientations.
JHLin, Applied EM; LCD
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Physical Principle
 Note that the molecules in contact with
the grooved surfaces align themselves in
parallel along the grooves, from a yorientation at the entrance substrate
into an x-orientation at the exit
substrate.
 The molecular spiral causes the crystal
to behave like a wave polarizer;
unpolarized light incident upon the
entrance substrate follows the
orientation of the spiral, emerging
through the exit substrate with its
polarization (direction of electric field)
parallel to the groove’s direction, the xdirection.
JHLin, Applied EM; LCD
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LCD Structure
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LCD Structure
 A single-pixel LCD structure is shown in Fig. TF14-2 for the OFF and
ON states, with ON corresponding to a brightlooking pixel and OFF to a
dark-looking pixel.
 The sandwiched liquid-crystal layer (typically on the order of 5
microns in thickness, or 1/20 of the width of a human hair) is
straddled by a pair of optical filters with orthogonal polarizations.
 When no voltage is applied across the crystal layer, incoming
unpolarized light gets polarized as it passes through the entrance
polarizer,
then rotates by 90 as it follows the molecular spiral, and finally
emerges from the exit polarizer, giving the exited surface a bright
appearance.
JHLin, Applied EM; LCD
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LCD Structure
 A useful feature of nematic liquid crystals is that their spiral untwists
under the influence of an electric field (induced by a voltage difference
across the layer).
The degree of untwisting depends on the strength of the electric field.
 With no spiral to rotate the wave polarization as the light travels
through the crystal, the light polarization will be orthogonal to that of
the exit polarizer, allowing no light to pass through it.
Hence, the pixel will exhibit a dark appearance.
JHLin, Applied EM; LCD
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Two-Dimensional Array
 By extending the concept to a two-dimensional array of pixels and devising
a scheme to control the voltage across each pixel individually (usually by using
a thin-film transistor), a complete image can be displayed.
JHLin, Applied EM; EMF Sensors
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Two-Dimensional Array
 For color displays, each pixel is made up of three subpixels with
complementary color filters (red, green, and blue).
JHLin, Applied EM; LCD
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